Thermal management in semiconductor devices and circuits is a critical design element in any manufacturable and cost-effective electronic and optoelectronic product, such as light generation and electrical signal amplification. The goal of efficient thermal design is to lower the operating temperature of such electronic or optoelectronic device while maximizing performance (power and speed) and reliability. Examples of such devices are microwave transistors, light-emitting diodes and lasers. Depending on the frequency of operation, power requirements, and specific application, these devices have been conventionally made on silicon, gallium arsenide (GaAs), or indium phosphide (InP). In recent years, gallium nitride (GaN), aluminum nitride (AlN) and other wide-gap semiconductors have surfaced as new choices for both power electronics and visible-light generating optoelectronics. Gallium nitride material systems give rise to microwave transistors with high-electron mobility (necessary for high-speed operation), high breakdown voltage (necessary for high power), and thermal conductivity that is greater than GaAs, InP, or silicon, and thus suitable for use in high power applications. GaN is also used in manufacturing of blue and ultraviolet lasers and light-emitting diodes. In spite of the high-temperature performance (owing to its wide bandgap and high critical field), GaN electronic and optoelectronic devices are limited in performance due to relatively low thermal resistance of the substrates commonly used for growth of GaN. This deficiency is most pronounced in high-power microwave and millimeter-wave transistors and amplifiers where reduced cooling requirements and longer device life, both benefiting from lower junction temperature, are in critical demand. Similar need is exhibited in high power blue and ultraviolet lasers where several-micrometer-wide laser cavity stripe dissipates power into the chip though low thermal conductance materials.
It is well known that diamond is the most thermally conductive substance known to man. For this reason, the semiconductor industry has been employing diamond heat-sinks and heat spreaders for improved thermal management since the commercialization of synthetic diamond by chemical-vapor deposition in the 1980's. The objective of optimal heat management is to bring the diamond heat-spreader or diamond layers to close proximity with the heat source in the electronic or optoelectronic devices. This means building devices on thin chips and mounted on diamond heat-spreaders, coating devices with diamond layers, and in more recent times transferring device epilayers onto diamond.
Diamond wafers are manufactured by chemical vapor deposition (CVD) by one of three methods: plasma enhanced diamond CVD where the energy to dissociate the reactants comes from a microwave source, hot-filament enhanced diamond CVD where the energy for dissociating gases comes from a hot tungsten filament, and high voltage torch where ions are accelerated using a high DC voltage.
The CVD process is carried out in a vacuum chamber within which a substrate on top of which diamond will be grown is provided and which is exposed to the energy source needed to dissociate the molecules of precursor gases needed to form diamond on the surface of the substrate. The precursor gases needed in the chemical vapor deposition of diamond are a source of carbon, typically methane (CH4), ethane (C2H6), carbon monoxide (CO), and acetylene (C2H2), diluted in hydrogen (H2). The gas combination needed for efficient diamond deposition contains a small (several percent) composition of the carbon-carrying gas in hydrogen, and the reaction can be further assisted with very small percentage of oxygen (O2). The preferred carbon-carrying gas in these reactions is methane.
GaN-on-diamond technology and resulting devices (described in U.S. Pat. No. 7,595,507) involve structures which feature atomically attached GaN epilayers to diamond substrates. This technology enables bringing together the best heat conductor (diamond) with electronic and optoelectronic devices based on gallium-nitride (GaN) and GaN-related compounds. Due to its inherent high critical electrical field and wide bandgap, gallium nitride devices are preferred for high power electrical and optoelectronic applications, such as, high power RF transistors and amplifiers, power management devices (Schottky diodes and switching transistors), as well as high power blue and ultraviolet lasers or light-emitting diodes.
GaN is presently grown on several different substrates: sapphire, silicon, silicon carbide, aluminum nitride, single-crystal diamond, and GaN substrates. With the exception of GaN substrates, all other materials have lattice constants that differ from that of GaN and AlGaN. In order to epitaxially grow high-quality AlGaN alloys on top of substrates with lattice constant different from GaN or the AlGaN alloys, it has been common practice in the industry to grow a layer or a combination of layers on top of the lattice-mismatched substrate in order to terminate the dislocations and produce a lower dislocation density epilayer on top of which growth of a high-quality active layer is possible. The layers grown directly on top of the lattice-mismatched substrate are commonly referred to as transition or nucleation layer or layers, and they can include any number of binary or ternary epitaxial layers. The nucleation layers are then followed by a suitably thick gallium-nitride layer, referred to as the buffer layer, which is added for achieving low dislocation density GaN and distancing the active layers from the highly dislocated nucleation layers. The top surface of the buffer layer generally features high quality material with dislocation density that is sufficiently low to allow the growth of device active layers. Typical dislocation densities achievable for GaN surface grown on silicon, silicon carbide, and sapphire epi-wafers for use in field-effect applications can be between 108 l/cm2 and 1010 l/cm2. Defect density required for efficient operation of bipolar devices, such as, bipolar transistors and optoelectronic devices ranges from 106 l/cm2 to 108 l/cm2. The GaN buffer layer is often required as a part of the active epilayer structure. For the purposes of this application, the term “transition layers” means all of the layers grown on top of the native substrate and these are needed to (a) convert the lattice constant of the native substrate to that of GaN, and (b) achieve high quality material, i.e. dislocation density low enough to allow the growth of the desired active layer on top.
The term device “active layers” refers to epilayer structure and structures required for realization of electronic devices such as a high-frequency transistor, high-voltage switch, Schottky diode, and/or optoelectronic devices such as laser diodes, light-emitting diodes, and super-luminescent diodes. As the GaN buffer layer may serve a function within the active layer and the transition layers, the boundary between the transition layers and the active layers may fall somewhere within the buffer layer. Its precise location may not be physical but functional: the boundary separates what is required for proper device functioning (active layers) from what is not required for device operation (transition layers).
The epilayer structure of a typical AlGaN/GaN HEMT shown in FIG. 1 (Prior Art) includes multiplicity of epilayers 9 disposed on top of a native substrate 1. The epilayers 9 are divided into two functional parts: the transition layers 8 and active layers 7. The transition layers 8 comprise of at least one layer, but typically a multiplicity of binary and ternary compound semiconductor epitaxial layers 2 that are grown directly on top of the native substrate 1, often referred to as nucleation layers, that are then followed by a buffer layer 3. The quality of the epilayers grown on the native substrate 1 improves as the growth progresses and at some thickness indicated with dashed line 17, the crystal quality (defect density) of the buffer 3 becomes sufficient for high-crystal quality growth of the active layer 7. The active layer 7 comprises multiple epitaxial layers whose number, thickness, and material choices are designed and optimized to perform specific functions of the electronic or optical device. The growth of the active layer 7 will start at and/or include growing a part of the buffer 3. The reason the active layer functionally includes a part of the buffer layer is because the 2DEG at a heterojunction extends into both the large and lower bandgap materials, and hence it is present in the GaN (as the lower bandgap material, namely, at top of the buffer). Furthermore, the active layer may include may include other features, such as, as the back barrier which would make it extend into the buffer.
It is essential to note that apart from providing a template for growth of the active layer, the transition layers generally do not serve a function in the device operation. From the point of thermal management, the presence of the nucleation layer 2 (part of the transition layer) is detrimental to device thermal performance. Namely, due to multiple layer-boundaries within the transition layer, alloy scattering, and dislocations, the transition layer generally presents a significant barrier to heat flow perpendicular to the layers and thereby limits the device thermal conductance.
The active layers 7 will typically comprise a barrier layer 6 on top of a layer structure 4 that may include a below-channel barrier (not shown) to reduce drain-induced barrier lowering as is well known in the art. The barrier layer 6 may furthermore include a several nanometer thick layer of GaN on top of the barrier layer 6 and/or an MN interlayer below the barrier layer 6 to improve the electron mobility in the two-dimensional electron gas 2DEG 5 as is also known in the art. The active layers 7 may comprise multiple layers of AlGaN or InGaN semiconductor alloys or GaN, AlN, InN or any other related material to realize the desired electrical performance of the HEMT. The buffer is needed to electrically separate the transition layers from the electron gas 5 and its thickness may be increased to improve the device breakdown voltage. The exemplary HEMT shown in FIG. 1 will also feature contacts to the transistor denoted with 10 (source), 11 (gate), and 13 (drain). The source 10 and the drain 13 contacts will typically make ohmic contacts to the active layers 7, while the gate 11 may make a Schottky contact to the active layer 7. Additionally, individual HEMTs may be isolated from adjacent devices on the same wafer or chip using isolation trenches 12 or implantation (not shown) to form monolithically integrated circuits on the same chip. The operation of this transistor and device enhancements described above have been described in publicly available literature, such as, books by Rüdiger Quay titled “Gallium Nitride Electronics”, and Umesh K. Mishra and J. Singh titled “Semiconductor Device Physics and Design”, both books published by Springer in 2008.
GaN-based HEMTs are used for numerous high power applications owing to the high density of electrons in the 2DEG in GaN and the high-breakdown field which lead to high operating currents and voltages, higher than GaAs devices of similar geometry. The dominant heat generation in high-electron mobility transistors occurs in an area between the gate and the drain 15, close to the device surface. In this area, the energy of electrons accelerated with the high drain potential are first converted into optical phonons by electron-phonon scattering and then by phonon-phonon scattering into acoustic phonons which carry heat (heat conduction). Conventionally, the HEMT shown in FIG. 1 is mounted with the back of the substrate 1 down onto a heat sink: The back metallization 16 is attached to a heat sink (not shown in FIG. 1). The heat generated in the active layers of the transistor has to diffuse to the backside of the wafer and be carried away through the backside 16 by the heatsink and dissipated in the ambient. The temperature rise of the active layer relative to the ambient temperature for a given power dissipated by the device is referred to as the thermal resistance and is an essential design parameter for all electronic devices as the active layer temperature determines the device performance and its reliability. It is the objective high-power electronic and opto-electronic design is to minimize the thermal resistance of any device and thereby improve their reliability and performance over temperature.
Thermal resistance of commercial HEMTs with exemplary structure shown in FIG. 1 is dominated by the relatively low thermal conductivity of the layers in the immediate proximity of the active layer, namely, the thermal conductivity of the active layers 7 and the transition layers 8. More specifically, the nucleation layers 2 which are a part of transition layers 8 may comprise ternary compound semiconductor alloys which exhibit low thermal conductivity. Finally, materials used commercially for the substrate 1 have low thermal resistance further contributing to the overall low thermal resistance of the devices (eg. sapphire, silicon). The result of these materials and structure limitations is that conventional AlGaN/GaN field-effect transistors are limited thermally, but could be made better if its the thermal resistance could be somehow reduced. In summary, the transition layers and the substrate thermal properties are liming the performance of the devices.
There is a need in the industry to improve the thermal performance of AlGaN/GaN HEMTs and similar high-power electronic and optoelectronic devices. This need has spurred a number of investigations in integrating wide-bandgap device active materials with highly thermally conductive substrates by wafer bonding and/or direct growth of wide-gap materials.
Related art references include U.S. Pat. No. 5,650,639 by Schrantz, et al., disclosing bonding of epitaxial layers with diamond substrates for the purpose for improving thermal performance; U.S. Pat. No. 7,033,912 Saxler teaching growing diamond on thinned silicon carbide substrates and optionally growing active layers on this structure; U.S. Pat. No. 6,794,276 Letertre, et al. teaching creation of new substrates for semiconductor devices; and U.S. Pat. No. 7,358,152 by Kub and Hobart disclosing a number of methods to improve the heat conductance of electronic devices, specifically, GaN HEMTs, using wafer bonding of either completed devices or blank GaN epiwafers to highly thermally conductive substrates, synthetic diamond included.